2000A&A...364..391L

We present results from 86GHz VLBI observations of 28 compact radio sources made in April 1993. All but two of the observed objects are active galactic nuclei. The remaining two objects are the galactic center Sgr A^* and the X-ray binary star Cygnus X-3. Of the observed sources, 26 have yielded fringe detections. We present correlated flux densities, estimate the maximum observed brightness temperatures, and provide single Gaussian component model fits. Out of the 17 sources with good uv-coverages, 3 are without detectable structural details (0642+449, 0716+714, and Cygnus A), 3 have little closure phase information (4C67.05, 1823+568, and 1928+738), and the remaining 11 have been imaged. We study the brightness temperatures, T_b, of the observed sources, and apply a basic population model with a single value of the intrinsic brightness temperature, T₀, in order to reproduce the observed distribution of T_b. Our data are consistent with a population of sources that have T₀≤5x10¹⁰K in the jets, and T₀∼1-4x10¹¹K in the VLBI cores. The observed T_b are also correlated with the apparent speeds measured in the jets. For comparison, we apply the same population scenario to a larger sample of 132 AGN observed with VLBI at 15GHz, and show that the distribution of brightness temperatures from that sample is consistent with the intrinsic brightness temperature of T₀=5.3x10¹¹K. The observed decrease of T_b along the jets can be reproduced by adiabatic losses in relativistic shocks embedded in the jet.